Slippery territory

It has taken just one year for the land-based biofuels (agrofuels) bubble to inflate and burst. In February 2007, the Ecologist was almost alone in pointing out that growing energy crops for car fuel was ecological nonsense; now the tide of academic studies showing that agrofuels are neither energy - nor carbon-efficient seems unstoppable.

Undeterred, the biofuels industry has picked itself up and is now pointing towards emerging technologies as replacements for agrofuels. One of these is the use of microalgae.

The idea of fuel from algae was first tested in the 1930s – but serious research did not begin until the oil crises of the 1970s, when the US Department of Energy (DoE) embarked upon the ‘Aquatic Species Program’ – a generously funded hunt for an algal strain that could grow quickly and produce large amounts of lipids to turn into biodiesel.

The programme was cut short in 1998, partly as oil prices fell, but mostly because the researchers had not managed to discover exactly what conditions would make algal cells pile on the pounds and swell up with lipid fats, the key ingredients of biodiesel.

Since the programme ended, however, a flurry of start-up companies have grabbed headlines claiming to have solved the problems of ‘algae-culture’. They are split into two main camps, which can be broadly described as the ‘algae fishers’ and the ‘algae farmers’.

The algae fishers plan on harvesting algae from naturally occurring sources and include Aquaflow – a New Zealand-based initiative that aims to produce biodiesel by recovering algae from sewage management ponds and farm settlement tanks. Because the algae require no special farming and clean up the water in which they grow, Aquaflow argues the process is cheap and environmentally friendly.

The problem is that naturally occurring strains of algae don’t tend to be suitable for making biofuel: they grow rapidly and tend to convert nutrients into carbohydrate and protein, rather than the all-important lipids. Dr John Benemann, one of the lead scientists in the original DoE research, is guarded about the potential of using naturally occurring strains of algae as an efficient energy source.

In a recent online article, he wrote: ‘There are thousands of [wastewater treatment pools] around, but they are mostly small and their algae have little or no oil – at least the way that we operate those systems at present. Making oil from algae grown on wastewaters still requires significant R&D.’

On top of this, research by Brussels-based environmental think-tank Biopact indicates that in some situations, harvesting algae grown in sewage ponds yields less energy than putting the sewage into a biodigester and burning the resulting methane gas.

Algae farmers have attempted to get around the low yields of naturally occurring algal blooms by controlling the environment in which specific strains are grown. Two techniques have been used: closed, temperature- and nutrient-controlled tanks known as ‘photobioreactors’ (PBRs), and open ponds through which algae are circulated. Both have met with difficulties, with PBRs proving expensive to build and ponds susceptible to contamination by naturally present, low-lipid algal species.

Massachusetts-based group GreenFuel Technologies uses PBRs. It bubbles waste flue gases from industrial facilities through the algae-filled reactors, providing the microscopic organisms with the nutrients necessary to swell quickly into fat-rich energy stores. Once harvested, the algae’s lipids are turned into biodiesel, its carbohydrates fermented into ethanol and its proteins into an animal feed.

But some scientists are sceptical about the financial and energy returns. Dr Krassen Dimitrov of the University of Queensland in Australia, an expert in biochemistry and molecular biology, is the author of a paper attacking GreenFuel’s technology. In a recent interview with Biopact, Dimitrov said:

‘With algae one has to always consider the trade-off between high growth-rates and how expensive it is to maintain conditions that would allow them. The other very important boundary is imposed by thermodynamics – the yield is limited by the amount of sunlight available – so improving the cultivating conditions follows the law of diminishing returns, as every percentage of yield that one can wrestle out becomes harder and harder as one approaches the theoretical limit.’

In December 2007, oil giant Shell announced an investment in Hawaiian firm HR Biopetroleum (HRB) Inc, whose scientists hope to get around the expense of bioreactors and the contamination problems of ponds by combining the two into a hybrid system. Algae are grown in the carefully controlled innards of a bioreactor then transferred to mature in large, lined ponds containing seawater. HRB claims to have produced an average of 9.5 tons of oil equivalent per hectare per year – greater even than the yield from a hectare of palm oil.

But as yet there are no reliable indicators of the energy balance of such facilities. HRB adds nitrate and phosphate fertilisers to its algae cultures, both energy-intensive to produce and transport, and uses chlorine to flush out its PBRs and ponds. Harvesting algae requires large amounts of energy: the algae must be collected, ‘spun’ in a centrifuge to remove water and dried in an oven at 60ºC for 24 hours. Once the oil and carbohydrates have been separated, an alcohol (usually toxic methanol) is used to produce the biodiesel.

Benemann describes open-pond techniques as ‘worthwhile trying’, but is sceptical about the overall impact algal fuels will have.

‘There are no silver bullets, no winner-take-all technologies, no technological fixes,’ he writes. ‘The solution to our energy and environment crisis can only come from, in order, “demand” management, efficiency improvements and new energy supplies, to which maybe algae processes can contribute.’